This invention relates to a process for depositing a layer of hydrogenated amorphous silicon (a-SiH) to a substrate, followed by a reaction of an unsaturated reagent to modify the surface properties including, but not limited to, inactivity to acidic, basic, and neutral compounds, resistivity to attack from caustic environments, and wettability toward other compounds. Radical quenching processes may also be used to complete the surface modification.
The current invention has been developed to overcome the inherent undesirable molecular activities of metal (ferrous and non-ferrous), glass, and ceramic surfaces which can cause the following: (a) chemisorption of other molecules, (b) reversible and irreversible physisorption of other molecules, (c) catalytic reactivity with other molecules, (d) allow attack from foreign species, resulting in a molecular breakdown of the surface, or (e) any combination of (a)-(d). The prior art shows the use of silanes or silicon hydrides to modify surfaces. The present invention utilizes the formation of a hydrogenated amorphous silicon coating on a surface through the decomposition of silanes or silicon hydrides, followed by a secondary process of surface functionalization with a reagent containing at least one unsaturated hydrocarbon group (e.g., xe2x80x94CHxe2x95x90CH2 or xe2x80x94Cxe2x89xa1CH). Additional elimination of residual surface defects can be achieved through reagents capable of thermal disproportionation and/or radical quenching.
It is known in the prior art to form a silicon hydride surface through a) halogenation of surface silanol moieties followed by reduction, and b) reacting surface silanol moieties with reagents such as trihydroxyhydridosilane via sol-gel type methods. This invention utilizes the thermal decomposition and deposition of silanes or silicon hydrides to impart an hydrogenated amorphous silicon layer on the substrate. The substrate does not require the presence of surface silanol moieties for the deposition to occur, thereby allowing a wide range of substrate types, such as metals, glasses and ceramics.
It is known in the prior art to functionalize a silicon-hydride surface with unsaturated hydrocarbon reagents in the presence of a metal catalyst. The complete removal of this catalyst from the treated system is often difficult and trace presence of the catalyst can reintroduce undesirable surface activity. The present invention does not employ an additional metal catalyst. Instead, the process is driven by heat. Without the use of a metal catalyst, the final product is void of additional residual catalyst activity and does not require removal of it.
It is known in the prior art to enhance the silicon hydride surface concentration by treatment of silicon metaloid with hydrofluoric acid. This invention utilizes the inherent formation of a silicon hydride surface via the thermal decomposition of silanes. Further treatment to enhance silicon hydride surface moieties are not necessary.
The present invention provides a modifying the surface properties of a substrate by depositing a layer of hydrogenated amorphous silicon on the surface of the substrate and then functionalizing the coated substrate by exposing the substrate to a binding reagent having at least one unsaturated hydrocarbon group. Using the method of the present invention, surface properties such as inactivity to acidic, basic and neutral compounds, resistivity to attack from caustic environments, and wettability toward other compounds is modified. The method can be used on ferrous and non-ferrous metal, glass and ceramic surfaces.
The method of modifying the surface properties of a substrate of the present invention comprises the steps of depositing a coating of hydrogenated amorphous silicon on the surface of the substrate, and then functionalizing the coated substrate by exposing the substrate to a binding reagent having at least one unsaturated hydrocarbon group under elevated temperature for a predetermined length of time. The hydrogenated amorphous silicon coating is deposited by exposing the substrate to silicon hydride gas under elevated temperature for a predetermined length of time. The pressure, temperature and time of exposure to hydrogenated amorphous silicon is controlled to prevent formation of hydrogenated amorphous silicon dust.
The substrate is initially cleaned by heating it at a temperature between about 100xc2x0 and about 600xc2x0 C. for a time period ranging from a few minutes to about 15 hours before exposing the substrate to hydrogenated amorphous silicon. In an embodiment, the substrate is cleaned at about 120xc2x0 C. for about 1 hour.
In a preferred embodiment, the substrate is isolated in an inert atmosphere before exposing the substrate to silicon hydride gas. The substrate is exposed to silicon hydride gas at a pressure between about 0.01 p.s.i.a. to about 200 p.s.i.g. and a temperature between about 200xc2x0 and 600xc2x0 C. for about 10 minutes to about 24 hours. More preferably, the substrate is exposed to silicon hydride gas at a pressure between about 1.0 p.s.i.a. and about 100 p.s.i.a. and a temperature between about 300xc2x0 and 600xc2x0 C. for about 30 minutes to about 6 hours. Better yet, the substrate is exposed to silicon hydride gas at a temperature between about 350xc2x0 and about 400xc2x0 C. for about 4 hours.
The pressure of the silicon hydride gas is preferably between about 2.3 p.s.i.a. and about 95 p.s.i.a.
In an embodiment of the invention, the substrate is exposed to silicon hydride gas at a temperature of about 400xc2x0 C. and pressure of about 2.5 p.s.i.a. In another embodiment, the substrate is exposed to silicon hydride gas at a temperature of about 40xc2x0 C. and a pressure of about 44 p.s.i.g., and then the temperature is raised to about 355xc2x0 C.
The substrate is also preferably isolated in an inert atmosphere before functionalizing the coated substrate with a substitute unsaturated hydrocarbon. The substrate is functionalized at a pressure between about 0.01 p.s.i.a. to about 200 p.s.i.a. and a temperature between about 200xc2x0 and 500xc2x0 C. for about 30 minutes to about 24 hours. More preferably, the substrate is exposed to the binding reagent at a temperature less than about 100xc2x0 C., and then the temperature is increased to between 250xc2x0 and 500xc2x0 C. while the pressure is maintained less than about 100 p.s.i.a.
The method may include the step of quenching residual silicon radicals in the hydrogenated amorphous silicon coating either before or after the hydrosilylation step. In this step, the substrate is isolated in an inert atmosphere before quenching the residual silicon radicals in the silicon coating. Preferably, the substrate is quenched exposing the substrate to organosilanes, amines, or known radical scavengers under elevated pressure and temperature for a predetermined length of time to cause thermal disproportionation. The substrate is quenched at a temperature between about 250xc2x0 and 500xc2x0 C. for about 30 minutes to about 24 hours.
The method may also include the step of exposing the substrate to oxygen prior to or contemporaneous with the hydrosilylation step. In one embodiment, up to about 5% by weight oxygen is mixed with said binding reagent. In another embodiment, the substrate is exposed to 100% oxygen or mixtures of oxygen in other gases at a temperature of about 100xc2x0 to 450xc2x0 C. for a few seconds to about 1 hour prior to hydrosilylating. More preferably, the substrate is exposed to zero air at about 25 p.s.i.g. and about 325xc2x0 C. for about 1 minute.